1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for processing image data, which is obtained by imaging such as radiation imaging, to display or output images.
2. Description of a Related Art
Conventionally, an imaging method using a radiation (X-ray, α-ray, β-ray, γ-ray, electron beam, ultraviolet ray and so on) has been utilized in various fields, and particularly, employed as one of the most important means for diagnosis in a medical field. Since a first X-ray photograph was realized, X-ray photography has been repeatedly improved. At present, a method using a combination of a fluorescent screen and an X-ray film has become a main stream. On the other hand, in recent years, various digitized apparatuses such as X-ray CT apparatus, ultrasonic imaging apparatus, and MRI apparatus are in practical use. And diagnosis information processing system is now under progression in hospitals. Also, as for X-ray images, many studies have been made for digitizing the systems, and a radiation imaging method using photostimulable phosphor has been established, which attracts attentions as a method that will replace the conventional X-ray photography.
The photostimulable phosphor (storage phosphor) is a substance which accumulates a part of radiation energy when irradiated with a radiation, and after that, emits stimulated fluorescence corresponding to the accumulated energy when irradiated with an excitation light such as visible light. The existence of the photostimulable phosphor is previously known. The radiation imaging method using the photostimulable phosphor will be described below. That is, by using a radiation imaging apparatus and a sheet which is applied with photostimulable phosphor, radiation imaging is carried out on an object to be imaged such as human body to record radiation image information on the photostimulable phosphor sheet. Then, using an image reading apparatus, the radiation image information is read out from the photostimulable phosphor sheet. That is, in the image reading apparatus, when the photostimulable phosphor sheet is scanned with an excitation light such as laser beam, photostimulable luminescent light is generated from the photostimulable phosphor. By detecting the light in a photoelectrical manner, the image data is generated. Further, after being appropriately processed with an image processing apparatus, the image data is output to a display such as a CRT, or printed out on a film with a laser printer or the like. Thus, the radiation image can be displayed as a visible image.
In view of imaging sensitivity and image quality, the radiation imaging method as described above has the performance comparable to the conventional X-ray photography. For example, compared to the conventional X-ray photography, the exposure latitude is extremely wider and the response of the photostimulable luminescent light with respect to the exposure intensity is substantially proportional all over the entire exposure range. Therefore, even when the object is imaged with any radiation dose, light-emitting area, where an image resides, can be covered and satisfactorily normalized to obtain digitalized signals. Further, by processing signals obtained as described above with an appropriate image processing method, images with satisfactory quality can be stably provided even under a variety of imaging conditions. Furthermore, since digitalized image information is directly obtained, large-scale data can be stored for a long time period without deterioration of the images and development to a medical diagnosis information system becomes possible.
The radiation imaging method as described above is applied to the radiation imaging of mammae (mammography), which is carried out in breast cancer diagnosis or the like. Visible images obtained by the radiation imaging are displayed on a monitor screen or printed out on a film to be used for medical diagnosis.
However, in the images obtained by the radiation imaging, a blank area having a high brightness occasionally occurs. FIG. 10 shows a mammograph. As shown in FIG. 10, a blank area 101 is generated along with a radiation image of a mamma area 100.
The reason why such blank area is generated in the mammograph is as described below. In the image reading apparatus, which reads out radiation image while conveying a sheet applied with photostimulable phosphor, a misalignment occasionally occurs on the sheet under transportation. In order to avoid such problem that the entire image data of a desired image area (for example, mamma area 100) cannot be obtained due to the misalignment during the conveyance, the start timing of the reading is adjusted to delay so that the entire image data of a desired image area is obtained. At that time, the adjustment to delay the start timing of the reading leads to such situation that an area out of the sheet is read out. That area is visualized on the film resulted in a transparent blank area 101. Since light is projected from the backside of the film when observing the radiation image, such blank area 101 appears as an area of a high brightness. Accordingly, there arises such problems that the visibility is reduced and eyestrain is caused.
As a related art, Japanese Unexamined Patent Application Publication JP-A-7-271972 discloses an image processing apparatus, in which at least an interested area in an output image is outputted at a desired density and/or tone in accordance with the purpose of diagnosis with an simple operation (pp. 4-6, FIG. 1). In this image processing apparatus, the density and/or tone in the radiation image can be set, and based on the set density and/or tone, a desired radiation image can be outputted. However, in the image processing apparatus disclosed in JP-A-7-271972, the desired interested area in radiation image to be displayed has to be set up by using a mouse, a keyboard or the like.
Further, Japanese Unexamined Patent Application Publication JP-A-9-97321 discloses the following image displaying method. That is, in an image displaying apparatus for displaying images, which are taken by use of an irradiation field stop, on a light-emitting display means such as CRT so that the brightness level of the area outside the irradiation field is higher than the brightness level of the area inside the irradiation field, reduction of contrast in the area inside the irradiation field, which is caused from scattered light from the area outside the irradiation field, is prevented (pp. 4-5, FIG. 1). In this image displaying method, as for the area where the image signal level is higher than a predetermined threshold value, the higher the image signal level becomes the lower the brightness level is set to, while as for the area where the image signal level is lower than the predetermined threshold value, the brightness level is set to a minimum brightness level. Thereby, the area outside the irradiation field, where the image signal level is low (the brightness level is high), is blackened to prevent contrast in the area inside the radiation field from being reduced.
However, in the mammograph, there is a possibility that a scattering area, where the brightness level becomes the same level as that of the area inside the irradiation field by reading out excitation light scattered by an edge of the photostimulable phosphor sheet, may reside in between the area inside the irradiation field and the area outside the irradiation field. Accordingly, even when the image displaying method disclosed in JP-A-9-97321 is applied to the mammography, the scattering area as described above cannot be blackened. Also, in this image displaying method, the area outside the irradiation field is detected only by comparing the image signal level with the threshold value. Accordingly, there is a possibility that the area inside the irradiation field where the image signal level is low (brightness level is high) might be blackened.
Furthermore, Japanese Unexamined Patent Application Publication JP-A-10-71138 discloses a radiation image processing apparatus, in which re-imaging is not required even when an error occurs in automatic recognition of area inside the irradiation field, and load of processing for recognizing the irradiation field is made smaller, while keeping anti-dazzle effect during observation of image (pp. 4-6, FIG. 3). In this radiation image processing apparatus, points, where the image data changes sharply to substantially zero, are determined as the boundary points between the area inside the irradiation field and the area outside the irradiation field. By connecting these boundary points, the boundary line is determined. And an external area of the boundary line is recognized as the area outside the irradiation field so as to perform the blackening processing.
However, in the mammograph, no points are included in some directions where the image data changes sharply to substantially zero from the center to the periphery thereof, as the mamma area 100 and the blank area 101 shown in FIG. 10. Accordingly, even when the radiation image processing apparatus disclosed in JP-A-10-71138 is applied to the mammography, there may be a case where the boundary line cannot be determined. According to this radiation image processing apparatus, however, in the case where a range of the area outside the irradiation field that has been automatically recognized is not true, the blackening processing can be made in an appropriate range by manually correcting the automatically recognized range while monitoring the image.